1. Field of the Invention
This invention relates to a change gear for automobiles, i.e., a helical gear, a device and a method for manufacturing the helical gear.
2. Description of the Prior Art
In manufacturing of a change gear to be used for a speed-changing transmission of an automobile, first, hobbing is performed as shown in FIG. 16, shaving is applied to a hobbed work, and a shaved work is subjected to heat treatment, whereby a gear is formed through the series of machining and treatment.
In the state before the heat treatment, the work includes internal stresses. The internal stresses released by the heat treatment cause heat treatment distortion and deform the work. The internal stresses are not uniform (equal), so that the deformation is complicated.
Therefore, accuracies before and after the heat treatment are measured and a shape in expectation of the deformation is fed back to the shaving cutter. However, a trial of the series of processes shown in FIG. 16, must be repeated a plurality of times. Alternatively, hard machining after the heat treatment is added. However, this increases the number of processes.
As shown in FIG. 17A, in conventional helical gears and helical gear manufacturing apparatuses and methods, in a die for working helical tooth portions of a helical gear as a work, when the work is pressure-forged with a punch by opposed working surfaces at a fixed distance formed so as to be inclined, worked surfaces on both sides of the helical tooth portions are ironed during the helical tooth portions passing through the die between the opposed working surfaces.
As shown in FIGS. 17A and 17B, in the above-described conventional helical gears and helical gear manufacturing apparatuses and methods, in a die for working helical tooth portions of a helical gear as a work, when the work is pressure-forged with a punch by opposed working surfaces at a fixed distance formed so as to be inclined, worked surfaces on both sides of the helical tooth portions are ironed during the helical tooth portions passing through the die between the opposed working surfaces. Therefore, in forge-forming, by using the spread of a material in the die under pressure, the material is spread to a predetermined size. However, as shown in FIG. 17B, the distribution of the internal stress strength and the direction thereof are not uniform and some portion shrinks due to buckling depending on the location. Therefore, in normal forging, a portion expanded under pressure and a portion shrunk by buckling are mixed. This increases heat treatment distortion.
As shown in FIG. 18, in the conventional manufacturing method using machining such as bobbing, grain flow of a material is cut by gear cutting, so that this also increases heat treatment distortion.
In the conventional manufacturing apparatus for forging by using a die shown in FIG. 17A to obtain a predetermined helix angle on the helical tooth portions, it is necessary that a helix angle and a helix angle correction amount of the work are obtained in advance as shown in FIG. 19 and a helix angle of the die is set in advance to be greater than that of the work.
Therefore, the inventor of the present invention focused on the fact that by making uniform, that is, equal, the vectors of internal stresses in the work before heat treatment, the heat treatment distortion could be minimized. Herein, “uniform” and “equal” are in both the stress direction (tension and compression) and the degree of stresses.
Therefore, the inventor of the present invention focused on the technical idea of the present invention in which, in a die for working helical tooth portions of a helical gear as a work, by lands formed on predetermined portions in tooth trace directions of opposed working surfaces formed to be inclined by making smaller a distance between opposed working surfaces than that of other portions, when the work was pressure-forged with a punch, worked surfaces on both sides of the helical tooth portions were ironed when the helical tooth portions passed through the lands. The inventor, as a result of repeated research and development, arrived at the present invention which realized the object to minimize the heat treatment distortion by making uniform, that is, equal, the vectors of internal stresses in the work.